The invention relates to an arrangement for closing roll nips in a multi-nip calender.
In a multi-roll calender (in the following also multi-nip calender) there often are as many as 10-12 rolls, which are located in the same or a different set of rolls in the same or a different frame so that one set of rolls always has 3-12 rolls. Each set of rolls has a first roll and a last roll, and one or more intermediate rolls between these rolls. In a set of rolls, a roll nip is always left between two adjacent rolls, in which roll nip the surface of the fibre web is profiled in a desired way. In a multi-roll calender, the roll nip is generally formed between a roll with an elastic surface, such as a polymer-coated roll, and a heated, smooth-surfaced steel roll or cast iron roll. For calendering both sides of the fibre web in the same way, the multi-roll calender often has a so-called reverse nip, which is a roll nip formed between two similar rolls, such as, for example, between two polymer-coated rolls. The one-sidedness of the fibre web can also be controlled so that, instead of the reverse nip, the calender is divided into two different sets of rolls. In a usual supercalender, in which the plane of the set of rolls is located substantially vertically in relation to the floor plane, the uppermost and the lowermost roll are variable crown rolls with chilled surfaces, in other words rolls, in which the deflection caused by their own weight has been compensated by internal loading elements of the roll. The intermediate rolls are alternately rolls with chilled surface, heated by water, and paper-or polymer-coated rolls; nowadays, most often polymer-coated rolls. The linear pressure in roll nips grows when transferring from the upper nip to the lower nip, due to earth gravity, and the linear loads of the roll nips depend on the specific weight of the rolls. The linear loads transverse to the machine direction of the roll nips, i.e. the linear loading profile also often has deflections, due to the load forces influencing the axle journals at the ends of the intermediate rolls, caused by auxiliary means, such as bearing houses and steam boxes.
In the so-called Optiload multi-roll calendering (multi-nip calendering) developed by Metso Paper, Inc., the own weight of the intermediate rolls has been lightened so that the axle journals are attached to loading arms: each intermediate roll is attached to loading arms from the bearing houses, the loading arms being again attached to the calender frame. With the loading arms it is possible to direct roll-lifting forces of different sizes to the ends of the roll and thus to compensate to a desired extent the influence caused by the own weight of the roll and the auxiliary means, loading the roll nips and thus increasing the linear loads of the roll nips. In this calendering method, also the deflections caused by the auxiliary means at the ends of the rolls have been compensated in the linear loading profiles transverse to the machine direction of the roll nips. The intermediate rolls have further been selected so that they have almost the same specific deflection caused by earth gravity. In this kind of calendering method, it is possible to use substantially the same linear pressure in all roll nips, i.e. the linear load distribution of the roll nips is uniform. Of the present calendering methods, this calendering method has the largest calendering window, i.e. with this method, it is possible to calender almost all paper qualities with high speeds while keeping the profiling quality of the paper good.
In the so-called Optiload method disclosed above, the lowermost roll is arranged to move on guide tracks in the calender frame, and the calendering is initiated by closing the roll nips above the lower roll by lifting the lowermost roll upwards in the plane of the intermediate rolls using hydraulic cylinders attached to the bearing houses. The additional load to the roll nips is brought either from above or below, for example, by loading the uppermost or lowermost roll with the additional load.